Bubbles in the oil level sight glass during air compressor operation need to be judged in two cases: “normal temporary appearance” and “abnormal continuous appearance”. In most cases, bubbles are a minor phenomenon in system operation, but continuous and massive bubbles may indicate potential problems. The core is related to “oil condition, system sealing, and circulation abnormalities”. Specific causes and countermeasures are as follows:
I. First, Distinguish: Normal Bubbles vs. Abnormal Bubbles
- Normal bubbles: Within 1-2 minutes of startup, a small number of fine bubbles appear in the oil level sight glass and gradually dissipate. The bubbles are uniform with no foam accumulation. This is a normal phenomenon where a small amount of air is introduced during the initial stage of lubricating oil circulation (especially for new machines or after oil change).
- Abnormal bubbles: A large number of dense bubbles (like “foamy water”) continuously appear during operation, or bubbles do not dissipate for a long time (more than 5 minutes). It may also be accompanied by abnormal oil level fluctuations and increased oil content in the exhaust. Focused investigation is required.
II. 5 Core Causes of Abnormal Bubbles and Solutions
1. Excessive or Insufficient Lubricating Oil Filling
The lubricating oil needs a certain space in the oil-gas separator for oil-gas separation. If the oil level is too high (exceeding the MAX scale), the oil-gas mixture will be intense during operation, and the oil will be stirred by high speed to trap a large amount of air, forming continuous bubbles. If the oil level is too low (below the MIN scale), the oil pump is prone to sucking in air when drawing oil (similar to how a water pump sucks in air when it can’t draw enough water), and the air mixes with the oil for circulation to generate bubbles.
Judgment: After shutting down the machine (wait 10 minutes to allow the oil to flow back to the oil separator tank), observe the oil level. If it exceeds MAX or is below MIN, it is an oil level problem.Solution: Drain excess oil or add special oil of the same model, and adjust the oil level between MIN and MAX.
2. Deteriorated Lubricating Oil or Contamination with Impurities
- Oil aging: After long-term use, the lubricating oil oxidizes and deteriorates, and its anti-foaming property decreases (the additives that originally inhibit bubbles fail), making it easy to form stable foam (bubbles are not easy to burst).
- Water contamination: Excessive condensed water in the air compressor (such as cooling system failure or humid environment) enters the oil. Under high temperature, it evaporates into water vapor and mixes with the oil to form bubbles (water is denser than oil; in severe cases, stratification or turbidity can be seen in the oil level sight glass).
- Mixing with other oils: Mixing lubricating oils of different models (such as mineral oil and synthetic oil) leads to poor oil compatibility, which destroys the stability of the oil film and generates bubbles.
Judgment: Observe the oil condition—aged oil is dark brown with a burnt smell; water-contaminated oil is turbid and emulsified (like “milk”); stratification or precipitation may occur after oil mixing.Solution: Completely replace with new oil (replace the oil filter at the same time). If water contamination is severe, first investigate the source of condensed water (such as cleaning the blowdown valve and checking for water leakage in the cooler).
3. System Air Leakage Leading to Air Mixing into Oil Circulation
If the lubricating oil circulation system (oil suction pipe, oil return pipe, joints) is poorly sealed, air will be sucked in and enter the oil separator tank with the oil to form bubbles. Common leakage points include:
- Loose oil suction pipe joints: When the oil pump draws oil from the oil separator tank, if the joint seal is not tight, air will be sucked in simultaneously (similar to how a straw sucks in air when leaking).
- Malfunction of the check valve in the oil return pipe of the oil-gas separator: The oil return pipe is responsible for delivering the separated oil back to the main unit. If the check valve seal is not tight, compressed air will rush into the oil return pipe from the oil-gas separator and flow backward into the oil chamber, generating bubbles.
- Worn main unit shaft seal: Aging of the main unit rotor shaft seal causes compressed air to leak into the oil chamber through the shaft seal gap and mix with the oil to form bubbles (often accompanied by increased oil content in the exhaust and oil mist at the air user end).
Judgment: After shutting down and relieving pressure, check if the oil suction pipe joints are loose (can be re-tightened); if oil is carried in the exhaust during operation, it may be a problem with the shaft seal or check valve.Solution: Tighten loose joints; replace aging check valves or shaft seals (to be operated by professional personnel).
4. Excessively High Oil Temperature or Poor Cooling
At high temperatures (exceeding 100℃), lubricating oil will accelerate oxidation, and its viscosity will decrease, making bubbles more difficult to burst (similar to how foam is more likely to form in hot oil). If the cooler is blocked or the fan malfunctions, leading to excessively high oil temperature, the bubble problem will be aggravated.
Judgment: Check the air compressor’s exhaust temperature gauge. The normal temperature should be ≤100℃. If it continues to overheat (such as above 110℃), the cooling is poor.Solution: Clean the dust (for air-cooled models with compressed air) or scale (for water-cooled models with descaling agents) on the cooler, repair the cooling fan, and ensure the oil temperature is controlled between 60-90℃.
5. Clogged or Failed Oil-Gas Separator
When the oil-gas separator filter element is clogged, the pressure inside the oil separator tank increases, and the resistance of lubricating oil circulation increases. The oil is “squeezed” under high pressure to generate turbulence, trapping air to form bubbles. If the filter element is damaged, unseparated compressed air directly mixes with the returned oil, also causing bubbles.
Judgment: Observe the pressure difference before and after the oil separator (normal ≤0.15MPa). If the pressure difference is too large (such as exceeding 0.2MPa), the filter element is clogged; if there is severe oil carry-over in the exhaust, the filter element may be damaged.Solution: Replace the oil-gas separator filter element and ensure it matches the air compressor model.
III. Summary: Bubbles Should Not Be Ignored—Timely Handling Prevents Hidden Dangers
A small number of temporary bubbles are not a cause for concern, but continuous and massive bubbles are often a sign of “abnormal oil, system air leakage, or component aging”. If left unaddressed, it may lead to: further reduction in the anti-foaming property of the lubricating oil and weakened lubrication effect (bubbles will damage the oil film); accelerated oil oxidation and shortened service life; and even overheating and wear of the main unit due to insufficient air intake.
During investigation, follow the order of “first check oil level → then inspect oil quality → finally check sealing”. Most problems can be solved by adjusting the oil level, changing the oil, or tightening joints. For complex faults (such as shaft seal or oil separator failure), it is recommended to contact professional personnel for maintenance.
Post time: Nov-14-2025
